The invention is directed to the provision of a process for reclaiming rare earth oxysulfide phosphors, especially yttrium oxysulfide, after screening of the phosphor on the faceplate of a color cathode ray tube. As used herein, a phosphor reclaim process is a series of chemical and/or physical steps which converts the excess phosphor collected from screen development, skirt trim and other steps in the manufacture of color cathode ray tubes into virgin quality powder which is available for reuse.
The high costs of rare earth phosphors such as yttrium oxysulfide has motivated phosphor makers and phosphor users to develop processes for reclaiming these phosphor materials from contaminated screening wastes collected during color cathode ray tube screening operations. A number of phosphor reclaim processes have been developed and placed into commercial use which are directed to purifying the reclaimed phosphor to a state of virgin quality. It has been found, however, that it is not sufficient merely to restore the purity of a used phosphor material by removing screening contaminants. The phosphor material must also be given good screening properties. The properties that cause a phosphor material to "screen" satisfactorily may be stated as follows:
1. The phosphor material must adhere to the surface of a cathode ray tube faceplate (glass). In "black surround" tubes, such as the CHROMACOLOR-brand tubes manufactured by the assignee of this invention, the phosphor material must also adhere to a layer used to form the "black surround" (typically graphite).
2. The phosphor material must have a high resistance to cross-contamination of earlier-deposited phosphor materials. "Cross-contamination" is herein intended to mean the adherence of particles of one phosphor material upon previously deposited phosphor elements having another color emission characteristic. For example, when the blue-emitting phosphor material, typically deposited subsequent to the deposition of the green phosphor material, is contaminated by particles of later-deposited red-emitting phosphor material, upon energization of the blue-emitting phosphor, its emission spectrum will be altered due to the presence of the red phosphor particles.
The property of a reclaimed rare earth oxysulfide phosphor material to adhere to its substrate during rescreening has been addressed by prior art reclaim processes. It is common practice to subject such reclaimed phosphor materials during a reclaim process to a wash in nitric acid in order to attack and cleanse the outer surface of the phosphor particles. The use of nitric acid to enhance the adherability of a reclaimed phosphor material has proven successful, however this process has a number of drawbacks:
1. Nitric acid is undesirably costly.
2. The high corrosiveness of nitric acid results in a relatively high yield loss in the reclaimed phosphor - for example, 3-4%, or more.
3. The use of nitric acid requires extensive afterwashing in order to adequately cleanse the reclaimed phosphor material.
4. Nitric acid causes the reclaimed phosphor material to clump, requiring an undesirable amount of blending and sifting and thus undesirable abrasion of the phosphor particles, in order to restore the phosphor material to a loose, powdery state.
The problems associated with the tendency of a reclaimed phosphor material to cross-contaminate have not been overcome by prior art processes. It is known that certain reclaims processes involve subjecting the reclaimed phospor material to a wash in ammonium carbonate in order to improve its resistance to cross-contamination. It has been found, however, that such prior art processes do not significantly affect the tendency of a reclaimed phosphor material to cross-contaminate.
Prior art phosphor reclaim processes have also been found to be deficient in their effectiveness in separating graphite contaminants from the phosphor material being reclaimed.
No prior art pertinent to this invention is known. The following patents disclose the use of basic compounds in connection with the treatment of phosphor materials or the screening thereof, however, their respective disclosures are conspicuously remote from the teachings of the present invention: U.S. Pat. Nos. 3,676,177; 3,568,629 and 3,523,904.